In the heat-producing electronic components such as the power device, a transistor, a thyristor, a CPU or the like, it has been an importance issue how efficiently to dissipate heat generated during use thereof. Conventionally, as such heat dissipation measures, the followings have been generally performed, (1) an insulating layer for a printed circuit board mounted with a heat-producing electronic component is made to have high thennal conductivity, (2) the heat-producing electronic component or the printed circuit board mounted with the heat-producing electronic component is attached to the heat-dissipating member such as a heat sink via an electrically insulating thermal interface material. As the resin composition for the thermal interface material or insulating layer for the printed circuit board, a silicone resin or epoxy resin filled with ceramic powders having high thermal conductivity is used.
In recent years, with rapid development of high-density mounting technology accompanying miniaturization of electronic devices, increase in speed and integration density of circuit inside the heat-producing electronic component, and increase in packing density of the heat-producing electronic component on the printed circuit board are in progress. Therefore, heat generation density inside the electronic devices has been increasing year by year, and a ceramic powder exhibiting higher thermal conductivity than ever has been required.
Under the background described above, hexagonal boron-nitride powder having excellent properties such as (1) high thermal conductivity, (2) high insulation property, and (3) low relative permittivity, as an electrical insulating material, has been attracting attention. However, a hexagonal boron-nitride particle has a thermal conductivity of 2 W/(m·K) in a thickness direction (c-axis direction), while a thermal conductivity thereof in an in-plane direction (a-axis direction) is 400 W/(m·K), and a thermal-conductivity anisotropy derived from a crystal structure and scale-shape is large (Non-Patent Document 1). Further, when the hexagonal boron-nitride powder is filled in a resin, the particles are all aligned in the same direction. Therefore, for example, during production of the thermal interface material, the in-plane direction (a-axis direction) of the hexagonal boron-nitride particle is perpendicular to a thickness direction of the thermal interface material, and thus high thermal conductivity in the in-plane direction (a-axis direction) of the hexagonal boron-nitride particle cannot be fully exploited.
In order to solve such a problem, Patent Document 1 proposes that the in-plane direction (a-axis direction) of the hexagonal boron-nitride particle is oriented in a thickness direction of a high thermal conductivity sheet. According to a technology proposed in Patent Document 1, it is possible to exploit high thermal conductivity in the in-plane direction (a-axis direction) of the hexagonal boron-nitride particle. However, the technology proposed in Patent Document 1 has problems that (1) it is necessary to stack oriented sheets in a next step, and thus a production process easily becomes complicated, and (2) it is necessary to cut the sheet into thin sheets after lamination and curing, and thus it is difficult to ensure dimensional accuracy in thickness of the sheet. Further, in the technology proposed in Patent Document 1, since the hexagonal boron-nitride particle is scale-shaped, viscosity is increased and fluidity is reduced at the time of being filled into the resin, and thus high density filling of the particles is difficult. In order to improve the problems, the hexagonal boron-nitride powders of various shapes, which have reduced thermal-conductivity anisotropy of the hexagonal boron-nitride particle, have been proposed.
For example, Patent Documents 2 and 3 propose to use the hexagonal boron-nitride powder which is aggregated so that hexagonal boron-nitride primary particles are not oriented in the same direction. According to a technology proposed in Patent Documents 2 and 3, it is possible to reduce the thermal-conductivity anisotropy. However, in the technology proposed in Patent Documents 2 and 3, the aggregated hexagonal boron-nitride powder has a pinecone shape (for example, see Patent Document 2: paragraph [0020] and FIG. 6) or a massive form (for example, see Patent Document 3: paragraph [0037] and FIGS. 3 to 5), and a mean sphericity thereof is small, and thus there is a limit to filling into the resin, and there is a limit to improvement of the thermal conductivity.
Further, Patent Document 4 proposes to use the hexagonal boron-nitride powder, which is borate particle coated with the hexagonal boron-nitride particle and has a high mean sphericity. According to a technology proposed in Patent Document 4, it is possible to obtain a certain effect for improving filling property into the resin and for reducing the theiinal-conductivity anisotropy. However, in the technology proposed in Patent Document 4, since the content rate of the borate particles having low thermal conductivity is high (for example, see paragraphs [0020], [0028]), there is a problem that it is not possible to fully take advantage of high thermal conductivity of the hexagonal boron-nitride particle.
Further, it is known that the thermal conductivity of the epoxy resin or the silicone resin is much lower than the thermal conductivity in the in-plane direction (a-axis direction) of the hexagonal boron-nitride particle. Therefore, the thermal conductivity of the resin composition, which is the resin filled with the boron-nitride particles of reduced thermal-conductivity anisotropy, is greatly affected by thermal contact resistance of interface between the resin and the boron-nitride particles. That is, in order to obtain the resin composition having high thermal conductivity, it is necessary to reduce the thermal-conductivity anisotropy of the boron-nitride particles, and to reduce the thermal contact resistance of the interface between the resin and the boron-nitride particles.
As a method for reducing the thermal contact resistance of the interface between the resin and the boron-nitride particles, it can be mentioned to (1) increase a mean particle diameter of the boron-nitride particles (reduce a total number of interfaces between the resin and the boron-nitride particles), (2) improve conformability between the resin and the boron-nitride particles by adding a silane coupling agent, and (3) bring the boron-nitride particles into surface contact with each other, however, an effect of (3) is greatest.
For example, Patent Documents 5 and 6 propose boron-nitride particles formed by isotropic aggregation of the hexagonal boron-nitride primary particles, and a thermally conductive sheet formed by dispersion of the boron-nitride particles in a thermosetting resin. According to the technology proposed in Patent Documents 5 and 6, it is possible to obtain a certain effect for improving filling property in the resin and for reducing the thermal-conductivity anisotropy. However, in the technology proposed in Patent Documents 5 and 6, since it does not consider reduction in the thermal contact resistance owing to the surface contact between the boron-nitride particles, there is a problem that it is not possible to fully take advantage of high thermal conductivity of the hexagonal boron-nitride particle.
Further, Patent Document 7 proposes a thermally conductive sheet characterized in that secondary particles composed of the boron-nitride primary particles are in surface contact. According to a technology proposed in Patent Document 7, it is possible to obtain a certain effect for reducing the thermal-conductivity anisotropy, for improving filling property in the resin, and for reducing the thermal contact resistance. However, Patent Document7 does not disclose to mix a sintering aid required for bonding the hexagonal boron-nitride primary particles (for example, paragraph [0017]), and a technology proposed in Patent Document 7 shows that porosity is less than or equal to 50 vol % (for example, paragraph [0014]), and thus it is difficult to obtain both easiness of deformation (low elastic modulus) and improvement in strength of the secondary particles. As a result, in the technology proposed in Patent Document 7, there is a limit to the improvement of the thermal conductivity by reducing the thermal contact resistance, and further technical development has been awaited.